In machine tools, when using a rotary tool to machine a workpiece, chatter vibration sometimes occurs. If chatter vibration occurs, it has a detrimental effect on machining, so to obtain high precision machining, chatter vibration has to be suppressed. As a method for suppressing chatter vibration, there is the method of making the pitch of the cutting edges of the rotary tool a variable one. Japanese Patent Publication (A) No. 2007-83329 discloses a rotary cutting tool by which a single tool can be used to form a variable pitch tool of various patterns. Further, Japanese Patent Publication (A) No. 2007-44852 discloses to detect chatter vibration and control the rotational speed of a spindle to a rotational speed where chatter vibration does not occur.
The prior art using a variable pitch cutting edge tool and the prior art disclosed in Japanese Patent Publication (A) No. 2007-83329 require the provision of a special tool with variable pitch cutting edges. There was the problem of increased costs of the tool. Further, the prior art disclosed in Japanese Patent Publication (A) No. 2007-44852 detects the chatter vibration to control the spindle rotational speed, so a chatter vibration detecting means becomes necessary. There was therefore the problem of increased costs of the machine tool.